Sorry for the response delay - this is not just the addon. This is happening in AdminCP too...
Scenario:
Admin 1 needs to add a secondary group to Admin 2
When Admin 1 goes into AdminCP and edits Admin 2's groups, the list only shows what they have access to, defaulting their primary group to the next available
We've tried enabling the Admins being allowed to add to admin groups but this gives them a lot more power than we are willing to give (i.e. root admin w/ unrestricted perms, web developer, etc...)